Hydrogen and carbon monoxide, which are important reactants in many processes in the chemical and petroleum refining industries, can be recovered from various gas mixtures containing hydrogen, carbon monoxide, methane, and inert gases such as nitrogen and argon. These gas mixtures can be generated by the steam reforming of natural gas or light naphtha followed by removal of water and carbon dioxide. Cryogenic distillation and pressure swing adsorption are well-known separation methods used to recover individual high-purity hydrogen and carbon monoxide products from these gas mixtures.
Hydrogen and carbon monoxide also can be recovered from certain offgas mixtures available in petroleum refineries and petrochemical plants. For example, offgas from acetylene production can contain these components in economically recoverable concentrations, but may contain low concentrations of ethane and heavier hydrocarbons in addition to hydrogen, methane, carbon monoxide, carbon dioxide, water, and inert gases. When ethane and heavier hydrocarbons are present in these offgas mixtures at certain concentration levels, modified separation steps may be required for recovering hydrogen and carbon monoxide from these mixtures.
The present invention, which is described below and defined by the claims which follow, addresses the recovery of hydrogen and carbon monoxide from gas mixtures containing these two components in admixture with methane and hydrocarbons heavier than methane.
The invention relates to a method for the recovery of hydrogen and carbon monoxide from a feed gas mixture containing hydrogen, carbon monoxide, methane, and hydrocarbons heavier than methane, which method comprises:
(a) separating the feed gas mixture containing hydrogen, carbon monoxide, methane, and hydrocarbons heavier than methane to yield an intermediate feed stream depleted in hydrocarbons and a reject stream enriched in hydrocarbons heavier than methane;
(b) separating the intermediate feed stream in a cryogenic separation system to yield a hydrogen-rich overhead product stream and one or more intermediate streams enriched in carbon monoxide and methane, wherein the separation system includes an absorption column refluxed with a methane-rich liquid reflux stream;
(c) introducing the one or more streams enriched in carbon monoxide and methane into a cryogenic distillation system and withdrawing therefrom a carbon monoxide-enriched overhead product stream and a liquid bottoms stream enriched in methane; and
(d) utilizing at least a portion of the liquid bottoms stream enriched in methane in (c) to provide the methane-rich liquid reflux stream in (b).
Separating the feed gas mixture in (a) may comprise cooling and partially condensing the feed gas mixture to yield a two-phase feed mixture, and separating the two-phase feed mixture to yield a vapor stream which provides the intermediate feed stream depleted in hydrocarbons and a liquid stream which provides the reject stream enriched in hydrocarbons heavier than methane.
The cryogenic separation system of (b) may include a partial condensation step in which the intermediate feed stream is cooled, partially condensed, and separated into a vapor feed stream and a liquid feed stream, wherein the vapor feed stream is introduced into the absorption column which is refluxed with a methane-rich liquid reflux stream, and wherein the liquid feed stream is introduced into the cryogenic separation system downstream of the absorption column.
The ratio of the molar concentration of hydrocarbons heavier than methane in the intermediate feed stream to the molar concentration of methane in the intermediate feed stream may be maintained at less than about 0.05, and may be maintained at less than about 0.02.
The invention also broadly relates to a method for the recovery of hydrogen and carbon monoxide from a feed gas mixture containing hydrogen, carbon monoxide, methane, and hydrocarbons heavier than methane, which method comprises utilizing a cryogenic methane wash column to recover hydrogen from an intermediate feed stream containing hydrogen, carbon monoxide, methane, and hydrocarbons heavier than methane, wherein the methane wash column may utilize a methane wash stream which contains less than about 5 mole % of hydrocarbons heavier than methane, and wherein the methane wash stream consists of components obtained from the intermediate feed stream. The methane wash stream may contain less than about 2 mole % of hydrocarbons heavier than methane.
The invention also relates to a system for the recovery of hydrogen and carbon monoxide from a feed gas mixture containing hydrogen, carbon monoxide, methane, and hydrocarbons heavier than methane, which method comprises:
(a) means for separating the feed gas mixture containing hydrogen, carbon monoxide, methane, and hydrocarbons heavier than methane to yield an intermediate feed stream depleted in hydrocarbons and a reject stream enriched in hydrocarbons heavier than methane;
(b) a cryogenic separation system for separating the intermediate feed stream to yield a hydrogen-rich overhead product stream and one or more intermediate streams enriched in carbon monoxide and methane, wherein the separation system includes an absorption column refluxed with a methane-rich liquid reflux stream;
(c) a cryogenic distillation system for separating the one or more streams enriched in carbon monoxide and methane into a carbon monoxide-enriched overhead product stream and a liquid bottoms stream enriched in methane; and
(d) piping means to transfer at least a portion of the liquid bottoms stream enriched in methane from the cryogenic distillation system to provide the methane-rich liquid reflux stream to the absorption column in the first cryogenic separation system.
The means for separating the feed gas mixture in (a) may comprise heat exchange means for cooling and partially condensing the feed gas mixture to yield a two-phase feed mixture, and a vapor-liquid separator for separating the two-phase feed mixture to yield a vapor stream which provides the intermediate feed stream depleted in hydrocarbons and a liquid stream which provides the reject stream enriched in hydrocarbons heavier than methane.
The cryogenic separation system of (b) may include heat exchange means for cooling and partially condensing the feed gas mixture to yield a two-phase feed mixture, a vapor-liquid separator for separating the two-phase feed mixture to yield a vapor feed stream and a liquid feed stream, piping means for introducing the vapor feed stream into the absorption column which is refluxed with a methane-rich liquid reflux stream, and piping means to introduce the liquid feed stream into the cryogenic separation system downstream of the absorption column.